1. Field of the Invention
This invention relates generally to processing image data obtained by scanning a document; and more particularly to discriminating between text material and continuous tone material in mixed format documents so that the image data may be appropriately processed.
2. Description of the Prior Art
Image processing for electronic copying or facsimile transmission is well know in the prior art. An original document is scanned to produce a video signal representative of the image. The video signal is next processed to optimize the reproduction.
Generally, text and low frequency halftone material (herein collectively referred to as text material) are processed with different algorithms than continuous tone and high frequency halftone material (herein referred to collectively as continuous tone material). Text material is optimized by a processing algorithm which results in sharp edge definition, high contrast, and high frequency detail rendition. Continuous tone material is optimized by an algorithm which results in good gray scale reproduction and only moderate detail rendition. The algorithms which are optimized for one type of material generally yield results which are marginal to poor on the other type of material.
Prior art methods to switch between algorithms optimized for text and those optimized for continuous tone reproduction include methods which discriminate based on the video signal level. The video signal is passed through a two-level comparator network. Signals above an upper threshold or below a lower threshold are processed by the text image processing algorithm, while signals falling between the two thresholds are processed by the continuous tone image processing algorithm.
While this system is generally accurate, small regions in text areas at the edges of objects would be found to be continuous tone regions. If the continuous tone image processing algorithm were used in such small regions, the printed edges would appear ragged. If the object were small, the entire object might be printed as continuous tone if the rise times of the input image are wider than the object. Also, low frequency halftones would have an undesirable appearance because the edges of the halftone dots would be considered to be continuous tone while the centers of the dots would be found to be text.